Introduce composed tree version of startOfDocument()

Source/core/editing/VisibleUnitsTest.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/editing/VisibleUnits.h"
 
 #include "core/editing/EditingTestBase.h"
 #include "core/editing/VisiblePosition.h"
 
@@ skipping 341 matching lines @@
     EXPECT_EQ(Position(one, 1), rightPositionOf(createVisiblePosition(Position(two, 2))).deepEquivalent());
     EXPECT_EQ(PositionInComposedTree(one, 1), rightPositionOf(createVisiblePosition(PositionInComposedTree(two, 2))).deepEquivalent());
 
     EXPECT_EQ(Position(five, 0), rightPositionOf(createVisiblePosition(Position(four, 4))).deepEquivalent());
     EXPECT_EQ(PositionInComposedTree(two, 0), rightPositionOf(createVisiblePosition(PositionInComposedTree(four, 4))).deepEquivalent());
 
     EXPECT_EQ(Position(), rightPositionOf(createVisiblePosition(Position(five, 5))).deepEquivalent());
     EXPECT_EQ(PositionInComposedTree(three, 0), rightPositionOf(createVisiblePosition(PositionInComposedTree(five, 5))).deepEquivalent());
 }
 
+TEST_F(VisibleUnitsTest, startOfDocument)
+{
+    const char* bodyContent = "<a id=host><b id=one>1</b><b id=two>22</b></a>";
+    const char* shadowContent = "<p><content select=#two></content></p><p><content select=#one></content></p>";
+    setBodyContent(bodyContent);
+    setShadowContent(shadowContent, "host");
+    updateLayoutAndStyleForPainting();
+
+    Node* one = document().getElementById("one")->firstChild();
+    Node* two = document().getElementById("two")->firstChild();
+
+    EXPECT_EQ(Position(one, 0), startOfDocument(createVisiblePositionInDOMTree(*one, 0)).deepEquivalent());
+    EXPECT_EQ(PositionInComposedTree(two, 0), startOfDocument(createVisiblePositionInComposedTree(*one, 0)).deepEquivalent());
+
+    EXPECT_EQ(Position(one, 0), startOfDocument(createVisiblePositionInDOMTree(*two, 1)).deepEquivalent());
+    EXPECT_EQ(PositionInComposedTree(two, 0), startOfDocument(createVisiblePositionInComposedTree(*two, 1)).deepEquivalent());
+}
+
 } // namespace blink